The present invention relates to cab signal equipment for a railway train and more particularly to a limit circuit thereof that is capable of producing an output signal proportional to rail current within a predetermined amplitude range.
In certain types of signal and communication systems used in rapid transit operations, it is common practice to employ cab signal receiving and decoding units to control train speeds within different track sections or restricted speed areas as the train moves along its route of travel. Generally the cab signals are conveyed to the train from the wayside in the form of frequency coded carrier waveforms. That is, a predetermined carrier frequency is selectively transmitted at different code rates corresponding to the desired speed at which the train is permitted or authorized to travel along a particular section of trackway. In practice, these coded carrier signals are normally fed to the track rails and are picked up by means of inductive coils mounted on the front end of the train. These induced frequency modulated signals are then filtered, amplified, demodulated, level detected and limited prior to being fed to several code frequency responsive networks comprising the decoding unit. Relay means responsive to the respective code frequency responsive networks control switches in brake control equipment shown and fully described in copending application Ser. No. 388,372, now U.S. Pat. No. 3,890,577. It will be understood, of course, that the number of these code frequency responsive networks is dependent upon the number of discrete speed levels provided for in the particular cab signal receiver equipment.
A block diagram of a state-of-the-art cab signal system is shown in FIG. 1 of the drawings where a cursory review will make it evident that a level detector circuit is employed in both the receiver and decoder units of the equipment.
In the receiver unit, the level detector functions to assure that the code signals received by the train are in fact valid signals transmitted via the rail and accordingly are not spurious or unwanted signals induced in the rail from a signal transmitted along an adjacent track section, for example. Experience has shown that rail current signals below a certain amplitude that is determined from a given multiple of the transmitted frequency may be considered invalid signals to be rejected. Because of the requirement that this level detector be of a "vital" design, i.e., one in which any malfunction of a critical component will fail to cause an output signal, a rather complex circuit has evolved, as shown in U.S. Pat. No. 3,614,466, which clearly shows and describes this circuit. In addition to the high cost of developing and building such a complex circuit, further expense results due to servicing requirements to adjust and maintain the circuit properly.
In contrast, the level detectors in the decoder unit monitor their respective code filter outputs to further assure that the signal amplitude falls within the frequency response bandwidth of the respective code filter with which each is associated, and accordingly are of considerably less complicated design than the level detector used in the receiver unit.